[vector] RFR: Remove VShiftV nodes

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/panama/vector/vshift/webrev.00/

Get rid of dedicated nodes for variable shifts and reuse existing 
[UR|R|L]ShiftV nodes.

Overall, it simplifies the code except detecting when variable shifts 
are supported. Introduced Matcher::supports_vector_variable_shifts() for 
that.

The patch is against vectorIntrinsics branch.

Best regards,
Vladimir Ivanov